The invention relates to improvements in tone wheels used to generate signals that correspond to the rate of rotation of a wheel or wheels of a land vehicle and pulse wheels used to generate signals that correspond to the angular position and speed of a combustion engine crankshaft.
It is known to provide a ferromagnetic toothed wheel or tone wheel to generate with a sensor an electrical signal that effectively instantaneously indicates rotational speed of a wheel in an automobile or other land vehicle. This information can be used to determine, among other things, that a particular wheel or wheels is/are skidding and to modulate the braking forces on the wheel or wheels to reduce or eliminate the skidding condition. A tone wheel can also be used in wheeled land vehicles to generate signals for a traction control system or in a transmission. Pulse wheels, in addition to signaling speed can also indicate angular position for example, in engine ignition control.
The accuracy of the signal depends, in part, on the accuracy of the formation of the teeth. It is important that the teeth are not only uniformly spaced about the periphery of a wheel, but also that the cross-sectional shape of each tooth is the same as all others and that the tooth profile is characterized by relatively sharp edges.
It is known to produce powder metal tone wheels. These prior art products have demonstrated that they are prone to accumulate oil, grease and debris between adjacent teeth. This accumulation, which can include metallic dust, diminishes the quality of the signal that the associated sensor can produce ultimately to a point where the anti-skid system fails to work properly. Foreign objects can become lodged between adjacent teeth on the tone wheel, which look like annular gears, and when such objects are carried past a sensor they may strike and break it or may abrade it to the point of destruction. An additional problem with powdered metal tone wheels is that they are susceptible to cracking in service in part due to residual stresses that occur when they are press-fit onto a shaft or other part. These cracked parts may produce error signals rendering the anti-skid system inoperative.
The invention, according to one aspect, provides an improved tone wheel fabricated from sheet steel in a configuration that produces an improved signal and is stronger, lighter and more cost effective than prior art products. Still further, the tone wheel of the invention reduces the risk of damage to the sensor associated with it.
As disclosed, the tone wheel has a multitude of evenly spaced axially oriented teeth at its periphery or rim. The teeth are supported in cantilever fashion from a wheel center portion so that the center portion has negligible influence on the signal being produced by the sensor. The tone wheel thus exhibits clear transitions between the presence and absence of a tooth in its scanned area.
More particularly, the tone wheel, formed from ferromagnetic malleable steel, assures that it will provide a long service life without risk of fracture through brittleness or fatigue. The inherent strength of the steel allows the wheel to be fabricated with relatively light gauge stock so as to reduce weight while retaining strength. The malleable steel is economical in cost and can be fabricated by relatively inexpensive stamping techniques.
In the illustrated forms of the tone wheel, the teeth are free of interconnected webs in the area scanned by the sensor. Consequently, since there is no structure between the teeth in the relevant areas, foreign material cannot readily build-up in the gaps between the teeth. Thus, there is a greatly reduced risk of damage to the sensor by abrasion or impact with foreign material which could otherwise accumulate or become lodged in the areas between the teeth. The open network of the teeth, as well as the improved signal generating character of the teeth, is augmented by an arrangement where the teeth are supported on roots which are formed on the outer periphery of the center portion of the wheel. The roots have intervening areas that are angularly aligned with the slots between the rim teeth. The resulting wheel configuration is devoid of any ferromagnetic material angularly between the teeth even at their juncture with the roots so that the open lattice or network of the teeth is maintained along their full length.
Another aspect of the invention involves both tone wheels and pulse wheels both hereinafter sometimes referred to as signal wheels that are made as composites of ferromagnetic material and non-magnetic material. This composite construction can afford still further reductions in weight, improvements in cost effectiveness and reduction in the risk of damage to the signal wheel and/or a sensor monitoring its rotation.
In the various disclosed embodiments of the composite signal wheel, the ferromagnetic material is formed into so-called xe2x80x9cteethxe2x80x9d evenly spaced from one another about the periphery of the wheel. The gaps between the spaced teeth are blocked by the non-magnetic or non-metallic material to reduce the risk that debris can be caught or collect between the teeth. As previously discussed, if debris would be caught or collected on the wheel, it can forcibly impact or eventually abrade and thereby destroy the sensor. Also, as previously mentioned in the case of an impact, the signal wheel itself can be damaged and rendered useless. In some embodiments, the non-metallic material is molded or otherwise formed so that it occupies the space between the teeth to produce a relatively smooth peripheral surface on the wheel with essentially no significant pockets between the teeth to catch debris. In other embodiments, the non-metallic material advantageously blocks the gaps between the teeth.
As disclosed, the non-metallic material such as a suitable plastic can be over-molded onto the ferromagnetic material such that it substantially completely encapsulates the teeth. In this manner, in addition to protecting the wheel from picking up debris, the plastic also protects the wheel against corrosion.
In pulse wheel versions of the signal wheel, typically, a single tooth on the wheel is omitted so that a sensor and associated electronics can determine the angular position as well as the speed of a shaft. When used inside an internal combustion engine of a car or truck, for example, the wheel may be partially immersed in a pool of oil. The non-metallic or plastic part of the wheel, by eliminating or reducing irregularities or pockets on the wheel, especially between the teeth, reduces the risk that the teeth can operate like paddles on a paddle wheel or vanes on an impeller. This reduces churning and aeration of the oil which can have adverse effects on other engine components.
Where desired, the non-metallic material can be configured to reinforce the metal wheel component thereby allowing the metal content to be reduced to only that required to develop a satisfactory signal. A reduction in the metal content can result in a reduction in weight and improvements in cost effectiveness both in material and processing.